The present invention relates to color ink compositions for ink-jet printing, and more particularly, to color ink compositions that enhance pen performance while minimizing color bleed.
The combination of low cost and high quality output have recently made ink-jet printers a popular alternative to other types of non-impact printers such as laser printers.
The ink-jet printing process involves the ejection of fine droplets of ink onto a print medium such as paper in response to electrical signals generated by a microprocessor. Typically, an ink-jet printer utilizes a pen set mounted on a carriage that is moved relative to the surface of a print medium. In commercially available ink-jet color printers, such as the DESKJET(trademark) printer available from Hewlett-Packard Company, a four-pen set including cyan, yellow, magenta and black inks is generally employed to achieve the necessary color combinations.
A typical pen includes print heads with orifice plates that have very small nozzles (typically 10-50 xcexcm diameter) through which the ink droplets are ejected. Adjacent to these nozzles are ink chambers where ink is stored prior to ejection. Ink drop ejection is currently achieved either thermally or piezoelectrically. In thermal ink-jet printing, each nozzle is associated with a resistor element. Each resistor element is in turn connected to a microprocessor, whose signals direct one or more resistor elements to heat up rapidly. This causes a rapid expansion of ink vapor that forces a drop of ink through the associated nozzle onto the print medium. In piezoelectric ink-jet printing, ink droplets are ejected due to the vibrations of piezoelectric crystals stimulated by electrical signals generated by the microprocessor.
Interactions between the ink and pen architecture (e.g. the resistor element, nozzle, etc.) strongly influence the reliability of pen performance. In addition, interactions between the ink and both the surface and bulk of the print medium play a key role in determining print quality. A significant amount of research has recently been conducted to produce improved ink compositions for ink-jet printers that exhibit favorable interactions with both the pen architecture and the print medium.
A variety of complex interactions between the ink and pen architecture can affect both the short and long term reliability of pen performance. For example, kogation, defined as the build up of residue on the surface of resistor elements as a result of repeated firings, can cause individual thermal heaters to fail, leading to a gradual degradation in pen performance.
Puddling and crusting relate respectively to the formation of ink puddles and insoluble crusts on the orifice plates of the printhead. Such obstructions lead to poor drop ejection characteristics (e.g. drop volume, velocity and direction), and hence to a degradation in print quality. Again, ink composition plays an important role in determining the extent of these two phenomena; the low surface tension of surfactant containing inks may cause puddling, while the evaporation of a volatile ink composition could lead to crusting.
In addition to the aforementioned properties affecting the reliability of the pens of a given pen set, a particular concern for color ink-jet printing, has been the mixing or xe2x80x9cbleedingxe2x80x9d that occurs both on the surface and within the print medium when inks of two different colors are printed side by side. Bleeding may cause undesired color formation at the interface (e.g. when cyan and yellow mix to give green) and a concurrent loss of color separation, resolution, and edge acuity. The more contrasting the two adjacent liquids are in color (e.g. black and yellow), the more visual the bleed. Several methods, including reducing dry times and increasing penetration rates, have been proposed to reduce bleed of adjacent printing liquids. In addition, pH-sensitive dyes may also be employed to control bleed.
U.S. Pat. No. 5,181,045 (incorporated by reference herein) discloses a method of ink-jet printing wherein one ink (a pH sensitive ink, usually a black ink) contains a colorant that becomes insoluble under defined pH conditions, and a second ink (the target ink, usually a color ink) has a pH that renders the colorant contained in the first ink insoluble. To completely control bleed, this method typically requires a pH differential of 4-5 units between the two inks. Accordingly, an ink with a pH not exceeding 4 would be preferred to effectively eliminate bleed from a pH-sensitive ink having a pH of 8.
U.S. Pat. No. 5,785,743 (incorporated by reference herein) discloses that the addition of an organic acid component to the so-called target ink composition reduces the pH differential required to control bleed to as little as 1-3 units. As a result, the pH of the target ink could be as high as 7 and still eliminate bleed from an encroaching pH-sensitive ink having a pH of 8, thereby reducing some of the corrosion risks associated with low pH inks.
The invention is an ink-jet ink composition. The composition comprises at least one colorant and a vehicle. The vehicle includes a mixture of succinic acid and at least one second organic acid. The second organic acid may be a monofunctional, difunctional or polyfunctional organic acid. The second organic acid may be glutaric acid, oxalic acid, maleic acid, methylsuccinic acid, malonic acid, adipic acid, fumaric acid, dihydroxyfumaric acid, malic acid, mesaconic acid, itaconic acid, phthalic acid, isophthalic acid, terephthalic acid, aspartic acid, glutamic acid, 1,2-, 1,3- and 1,4-cyclohexane dicarboxylic acids, 1,2,3-cyclohexane tricarboxylic acid, 1,2,4-cyclohexane tricarboxylic acid, 1,3,5-cyclohexane tricarboxylic acid, 1,2- and 1,3-cyclopentane dicarboxylic acids, citric acid, tartaric acid, dihydroxyterephthalic acid, 1,2,3-, 1,2,4- and 1,2,5-benzene tricarboxylic acids, tricarballylic acid, 1,2,4,5-benzene tetracarboxylic acid, norbomene tetracarboxylic acid, 3,3xe2x80x2, 4,4xe2x80x2-benzophenone tetracarboxylic acid, 1,2,3,4,5,6-benzene hexacarboxylic acid, acetic acid, polyacrylic acid, glycolic acid, and derivatives thereof. Preferably the second organic acid is glutaric acid.
The concentration of succinic acid may be from about 2 to about 8 wt %, for example from about 3 to about 6 wt %. The concentration of glutaric acid may be from about 0.1 to about 4 wt %, for example from about 0.5 to about 1.5 wt %.
The vehicle may further include from about 0.1 to about 7 wt % surfactants and from about 5 to about 25 wt % organic cosolvents.